A field effect transistor for detecting gas

Abstract

A FIELD EFFECT TRANSISTOR FOR DETECTING GAS ABSTRACT A Field Effect Transistor for detecting gas comprises a dielectric positioned between a gate electrode stacking and a semi conductor material to provide insulation. A high dielectric constant material is positioned in the gate stacking to increase gate capacitance and reduce leakage current A temperature sensitive catalytic metal is positioned on the gate stacking to react with a Phosphine gas detected. Figure:01

Description

FIELD OF INVENTIONThe present invention relates to a gas sensor. More particularly, the present invention relatesto a highly sensitive stack Engineered Junction less Accumulation Surrounding Gate FET(SE-JAM-SG-FET) for PH3 gas detection.BACKGROUNDThe subject matter discussed in the background section should not be assumed to be prior artmerely as a result of its mention in the background section. Similarly, a problem mentioned inthe background or associated with subject matter of background section should not beassumed to have been previously recognized in the prior art. The subject matter in thebackground section merely represents different approaches which in and of themselves mayalso correspond to implementations of claimed technology.FET type gas sensors are rapidly gaining popularity due to their lower power consumption,compact size, high reliability and compatibility with CMos technology. Existing FET sensorsmade FET based on ZnO nanorwires which operates on a principle of MOSFET and hascharge carrier channel made between source and drain.Present invention is different from various known devices which had just two electrodescathode and anode. In a process of sensing a gas by a semiconductor gas sensor, the gas isadsorbed on a surface of semiconductor. If an electron affinity of the gas molecule is largerthan a work function of semiconductor, the electrons on the semiconductor surface would betransferred to the gas molecular, so the latter would become cathodic ion. Such a gas likeO2 and NOx is called oxidizing gas. Reversely, if the electron affinity of gas molecular issmaller than the work function of semiconductor, the electrons on the gas molecular would betransferred to the semiconductor so the former would become anodic ion. Such a gas like H2,CO, and C2H5OH is called reducing gas. In a carbon monoxide gas sensor, a sensing materialtypically is a SnO2. When carbon monoxide comes in contact with the SnO2 surface aconductivity of semiconductor changes. Magnitude of a change is proportional to the changeof CO gas in environment. In another example a tungsten oxide is used as the sensingmarterial , resulting in a shift in a threshold voltage and drain current of the MOSFET. Themagnitude of the shift is proportional to a concentration. In another aspect hydrogen sensorsare used. The exisiting gas sensors have used metal oxide nanowires . However, usage of metal oxide nanowires involve some limitations which have not been adderessed in the priorarts, limations are material stability leading to reduced sensitivity and reliability, fabricationcomplexity since it requires precise nanowire synthesis, doping and integration withMOSFET, high power consumption to maintain sensor at optimal temperature, acheieving arapid recovery after exposure to gases can be challenging. The problems due to variation invapour pressure and adsorption rate of the gas due to room temperature has not beenaddressed in the prior arts.OBJECTS OF THE INVENTION:A general objective of the present invention is to give a PH3 (Phosphine) gas sensor.Another objective of the present invention is to give a sensitive stack Engineered Junctionless Accumulation Surrounding Gate Field Effect Transistor for sensing phosphine gasdiffusing into the Gate Electrode of FET.Yet another objective of the present invention is to give an increased gate capacitance and reduced leakage current of the gas sensing Field Effect Transistor.Yet another objective of the present invention is to give a simulated study of sensing thephosphine gas at different temperatures (100 K,200 K,300 K & 400 K).SUMMARY OF THE INVENTION:This summary is provided to introduce aspects related to a Field Effect Transistor fordetecting gas. A behavior of various catalytic materials when exposed to PH3 (phosphine) gas is disclosed.In an aspect, a dielectric positioned between a gate electrode stacking and a semiconductormaterial to provide insulation. A high dielectric constant material positioned in a gatestacking to increase gate capacitance and reduce leakage current. A temperature sensitivecatalytic metal positioned on the gate stacking to react with a gas detected.In an another aspect, the Field Effect Transistor for detecting gas is a highly sensitive stackEngineered Junction less Accumulation Surrounding Gate Field Effect Transistor.In yet another aspect, the field effect transistor detects a PH3 (Phosphine) gas.In yet another aspect, the Field Effect transistor for detecting gas uses Iridium as the catalytic metal.In yet another aspect, the Field Effect Transistor for detecting uses silicon dioxide (Sio2).asdielectric positioned between the gate electrode stacking and the semiconductor material.In yet another aspect, the Field Effect Transistor for detecting gas uses Hafnium (IV) oxide (HFo2) as a high dielectric constant materialBRIEF DESCRIPTION OF THE DRAWINGSThe accompanying drawings constitute a part of the description and are used to provide afurther understanding of the present invention.Fig.1 illustrates a Cylindrical SE-JAM-SG FET in 3-D structural view, with respect to anembodiment of the present inventionFig.2(a) illustrates a 2-D cross-sectional view of a designed SE-JAM-SG FET, with respectto an embodiment of the present inventionFig 2(b) illustrates a catalytic gate metal of the designed SE-JAM-SG FET with PH3 gassensor is exposed to phosphine gas, with respect to an embodiment of the present invention.Fig.3 illustrates a change in a drain to source current (Ids) with respect to gate to sourcevoltage (Vgs) of SE JAM SG FET with iridium, pd, pt and Rh as a gate catalytic metals.Fig.3 (a) to 3(d) illustrates a change in Ids with respect to Vgs for all catalytic metals atdifferent temperature ranges 100 K,200 K, 300 K & 400 KFig.4 illustrates a comparison of four distinct catalytic metals (Pt, pd, Rh, and Ir) for theproposed SE-JAM-SG FET with respect to the Off-State current for phosphine gas detectionat room temperature (300 K) with respect to an embodiment of the present invention.Fig.5 illustrates an Ion / Ioff ratio of a PH3 gas sensor using four catalytic metals (Rh, Pt, Rh,and Ir) for different temperature change (T=100 K,200 K,300 K & 400 K) with respect to anembodiment of the present inventionFig.6 illustrates a threshold voltage with respect to temperature modification for differentcatalytic gate metals with respect to an embodiment of the present inventionDETAILED DESCRIPTIONThe detailed description set forth below in connection with the appended drawings isintended as a description of various embodiments of the present invention and is not intendedto represent the only embodiments of the present invention and is not intended to representthe only embodiments in which the present invention may be practiced. Each embodimentdescribed in the disclosure is provided merely as an example of the illustration of the presentinvention, and should not be construed as preferred or advantageous over other embodiments.The detailed description includes specific details for the purpose of providing a thoroughunderstanding of the present invention. However, it will be apparent to those skilled in the artthat the present invention may be prescribed without specific details.Gas sensors have already utilized in field-effect transistors (FETs) based on metal oxide nanowires, such as ZnO, In2O3, and SnO2. A different class of FET-based gas sensors that employcatalytic metal gates is described in earlier works.In the present invention, a high-k dielectric gate stacking material called HfO2 is used, owingto increasing gate capacitance and lowering leakage current to improve carrier conductionthrough a channel. A gate-metal-work function changes as a result of a reaction between thegate metal and the gas, and this change depends on how much phosphine is adsorbing on thegate electrode surface. Proposed device's properties are simultaneously impacted by thechange in the gate-metal-work function.Therefore, a calculation of the device's gas sensitivity takes into consideration the change inthe device's Off-state current (IOff), On-state current (IOn), and On- state to Off- state current(IOn / IOff). A proposed and simulated study is analyzing the phosphine gas at differenttemperatures (100 K,200 K,300 K & 400 K). Temperature research is for industrialapplication for temperature-sensitive electronic equipment's in the military, automobiles,nuclear power, communication systems, spacecraft, photo detectors, and terrestrial systems.Since variation in room temperature affect the phosphine gas's vapor pressure and adsorptionrate. So, temperature sensitivity is considered an important parameter for device performancefor various catalytic metals.ANALYTICAL ANALYSISPoisson's equation in cylindrical coordinates is stated as:Equaction 1Here φ(r, z, T)-> -> potential distribution where r-> radial coordinate, z-> channel length coordinate, Nd-> channel doping concentration, ϵsi-> permittivity ofSiand T-> temperature.The electrostatic potential in SE-JAM-SG FET is given by:φ(r, z, T) = V(r, T) +U(r, z, R) …. (2)Where V (r,T) is the solution of 1-D Poisson's equationEquaction (3)U(r,z,T) is the solution of 2-D Laplace equationEquaction …. (4)A change in a catalytic metal's work function caused by a reactivity of gas molecules at a metal surface is denoted by symbol ∆Φm. Flat band voltageVfb is given by:Vfb(T) = Φm - Φs(T) ±∆ Φm …. (5)Where Φs(T) is a silicon work function & can be defined byEquaction …. (6)Eg is a energy band gap, X is electron affinity, ∆ Φmis controlled by the gate metal, the gas to be detected and can be described by:Equaction …. (7)where P is the gas partial pressure, X is the gas constant, and F is Faraday constant. Since a concentration of gas molecules determines partial pressure, gas sensors can be calibrated to read in moles of gas per mole of air.Boundary conditions to solve 1-D Poisson's equation is denoted by:(i) Center potential function of z:φ(ρ,z,T)|ρ=0 =φc(z,T) =C0(Z) ....(8)(ii) Electric field at center:Equaction ...(9)(iii) Electric field at silicon oxide interface:Equaction ....(10)Equaction toxeff ,thk are thickness of effective gate dielectric layer & high-k, HfO2 respectively.Cox is capacitance per unit area of oxide.Solution of the 1-D Poisson's equation can be expressed using the boundary conditions (8-10) mentioned above as:Equaction ...(11)SE-JAM-SG FET has higher doping concentration in Source / Drain regions compared tochannel region. So, build in potential is given as:Equaction ....(12)k, q represents boltzmann's constant, electron charge.Using boundary conditions given as:(iv) Potential at z=0ϕ(r,0,T) = Vbi ....(13)(v) Potential at z=Lϕ(r,z = L,T) = Vbi+Vds ....(14)Solution of 2-D Laplace equation is given as:Equaction …. (15)Jo, J1 is Bessel functions of an order of zero & one respectively. λn is an eigenvalue for:Equaction … (16)Total center potential obtained asEquaction ....(17)Where A1 and B1 are Bessel-Fourier series coefficients are given as:EquactionWhereEquactionAnd Vgs' = Vgs - VfbCharacteristics and performance of the device are impacted by temperature.Several temperature-sensitive parameters include:Band gap energy (Eg):Equaction …. (18)Where, Eg (300) = 1.08 eV, Egα = 4.73 * 10-4 eV / K, α and γ are fitting parameters.γ = 636 KIntrinsic carrier concentration ηi(T):Equaction …. (19)Permittivity of silicon:εSi(T) = 11.40 + (1 + 1.2 x 10-4T) …. (20)Mobility is a significant parameter that is also impacted by temperature. For modellingmobility while taking the impact of temperature into account, Caughey and Thomas' function is represented as:Equaction ....(21)Where μ1=35.22 cm2 / Vs, m = -2.32, n=-3.80μ2 = 240 cm2 / Vs, Ncrit = 1.073 x 1017 cm -32The electric field E (ρ, z, T) distribution in the channel is calculated by differentiating the center potential as:Equaction …. (22)Drain current of a device is sum of all current at different gate voltages is given as:Equaction …. (23)Current across the source and drain of a MOSFET in subthreshold region / weak-inversionregion is defined as Subthreshold current (ISub) for 0 ≤ Vgs ≤ Vth. It is given as:Equaction …. (24)Where μ = 1300 cm2 / Vs is the electron mobility, k ->Boltzmann's constant (1.38x10-23J / K).Linear current can be given as:ILin (Vgs, Vds,T)= β[(Vgs - Vth)Vds]... (25)Equaction ....(26)The gate voltage, effective channel length & width are represented by Vgs, L, W respectively.Vds& Vth represents the drain to source voltage and threshold voltage respectively.ISat is formulated as:Equaction …. (27)Trans conductance is determined by:Equaction …. (28)Output conductance is defined asEquaction …. (29)Equaction …. (30)Equaction …. (31)Fig. 1 illustrates a SE-JAM-SG FET's 3-D structure, wherein three terminals of the FieldEffect Transistor, ie. gate, drain and source is illustrated. A dielectric SiO2 is placed betweenthe semiconductor and the gate electrode to provide insulation. A high dielectric materialHafnium (IV) oxide HfO2 is placed in the Gate Stacking to increase a gate capacitance valueand reduce a leakage current.Fig. 2(a) illustrates a cross-sectional visualization in 2-D. Along with SiO2 gate dielectric, thegate stack also contains the high-k material HfO2, which improves carrier conduction via the channel by reducing gate leakage. The diffusion leads to a reaction between catalytic gate metal and the gas.Fig. 2(b) illustrates a catalytic gate metal of the designed SE-JAM-SG FET, PH3 gas sensor isexposed to phosphine gas. Along with the SiO2 gate dielectric, the gate stack also contains thehigh-k material HfO2, which improves carrier conduction via the channel by reducing gateleakage. A diffusion of PH3 gas molecules is observed on catalytic gate metal leading to achange in a work function of the catalytic gate metal. The work function depends on theamount of gas that is diffused. The Iridium material having a high dielectric constant is usedas it helps in increasing the gate capacitance and lowering the leakage current. The catalyticmaterial Iridium is highly sensitive to temperature changes. A various simulation carried outwith various parameters for the other catalytic metals (Pd, Pt, and Rh) gives a comparativestudy of the highly sensitive catalytic metals.Table 1 : The physical parameters that were used in the simulation are listed below:Table 2: Various models used for simulation set up and their respective description.Fig.3 illustrates a change in drain to source current (Ids) with respect to gate to source voltage(Vgs) of SE JAM SG FET with iridium, pd, pt and Rh as gate catalytic metals. The results areobtained for all catalytic metals at Vds= 1.0 V, (b) Iridium (Ir), (c) Palladium (Pd), (d)Platinum (Pt). & (e) Rhodium (Rh) for temperature 100 K, 200 K, 300 K & 400 KIridium requires a highest energy to remove an electron from Fermi level of the metal to apoint outside the metal surface, leading a catalytic metal to reduce both the ON state currentand the OFFstate current.Fig.3 (a) to 3(d) illustrates a change in Ids with respect to Vgs for all catalytic metals atdifferent temperature ranges 100 K,200 K, 300 K & 400 K. Iridium based SE-JAM- SG-FETexhibits less current fluctuation with temperature change than other catalytic metals due tovariation in carrier mobility brought by a change in Fermi level.Fig.4 illustrates a comparison of four distinct catalytic metals (Pt,pd,Rh, and Ir) for the proposed SE-JAM-SG FET with respect to the Off-State current for phosphine gas detection at room temperature (300 K) with respect to an embodiment of the present invention. The work function causes OFF state current (IOff) to change when molecules of PH3 gas are exposed on the surface of the catalytic metal gate. This modification resulted in the overlapping of energy bands at junction of drain channel, which lowers Off-state current even more. As a result, sensitivity is also improved as formulated in equation:SIoff = Ioff(before gas absorption) / Ioff(after gas absorption)Furthermore, 200 meV change in work function at room temperature is also studied andobserved that Ir has improved sensitivity compared to Pd, Pt & Rh, whereas Rhodiumexhibits lowest sensitivity as compared to other catalytic metals in SE-JAM-SG FET for PH3 gas detection.Fig.4 also illustrates a comparison between the sensitivity of On-State Current for differentcatalytic metals (Rh, Pt, Pd, Ir) used for PH3 gas detection with respect to an embodiment ofthe present invention. Majority of carriers move from source to drain with an increasedsaturation velocity as a result of the work function change, which also raises the Ion sensitivityas determined by Equation:SIon = Ion(after gas absorbtion)-Ion(before gas absorbtion) / Ion(before gas absorbtion). Iridium exhibits enhanced sensitivity compared to other catalytic metals for 200 meV workfunction change. Iridium is therefore the catalytic metal with highest sensitivity among all of them as it has a higher work function.Fig.5 illustrates an Ion / Ioff ratio of a PH3 gas sensor using four catalytic metals (Rh, Pt, Rh,and Ir) for different temperature change (T=100 K,200 K,300 K & 400 K) with respect to anembodiment of the present invention. Due to the fermi level and conduction band overlapping,a large number of inverted mobile charge carriers are produced in channel with work functionvariation. More band bending is caused by this overlap at the channel-drain junction region,which helps the flow of charge carriers to the drain terminal, As a result, both Off-current andthe On-current rises. For each catalytic metal, the overall ratio of IOn / IOff consequently rises.When compared to Palladium, Platinum & Rhodium, Iridium exhibits higher IOn / IOff currentratio. For any semiconductor device, Ion / Ioff Sensitivity can be characterized as:EquactionFig.6 illustrates a threshold voltage with respect to temperature modification for different catalytic gate metals with respect to an embodiment of the present invention. Threshold voltage increases as the temperature rises. Also, as threshold voltage is directly proportional to the gate-metal-work function. The threshold voltage rises the most when Iridium is used because it has a larger work function than Pd, Pt and Rh. Higher threshold voltage is needed to accommodate extra positive ions that have accumulated in depletion region close to channelgate oxide contact. A Flat band voltage, which changes in accordance with change in the metal-work function, influences threshold voltage. Variation in Ioff is caused by a change in sub threshold current, which depends on the threshold voltage. Also, it can be observed from Fig. 6 that at 400 K (maximum temperature) maximum threshold voltage is obtained.Although implementations of the Field Effect Transistor for detecting gas has been described in language specific to structural features and / or methods, it is to be understood that the appended claims are not necessarily limited to the specific features or methods described. Rather, the specific features and methods described as examples of implementations of the Field Effect Transistor.

Claims

1. A Field Effect Transistor for detecting gas comprising: a dielectric positioned between a gate electrode stacking and a semiconductor material to provide insulation; a high dielectric constant material positioned in the gate stacking to increase gate capacitance and reduce leakage current; and a temperature sensitive catalytic metal positioned on the gate stacking to react with a gas detected.

2. The Field Effect Transistor for detecting gas as claimed in claim 1, wherein the Field Effect Transister is a highly sensitive stack Engineered Junction less Accumulation Surrounding Gate Field Effect Transistor.

3. The field effect transistor for detecting gas as claimed in claim 1, wherein the gas is PH3 (Phosphine) gas.

4. The Field Effect transistor for detecting gas as claimed in claim 1, wherein the catalytic metal is Iridium.

5. The Field Effect Transistor for detecting gas as claimed in claim 1, wherein the dielectric positioned between the gate electrode stacking and the semi conductor material is silicon dioxide (SiO2).

6. The Field Effect Transistor for detecting gas as claimed in claim 1, wherein the high dielectric constant material is Hafnium (IV) oxide (HFo2).